Microelectronic devices, such as integrated circuit chips, are becoming smaller and more powerful. The current trend is to produce integrated chips which are steadily increasing in density and perform many more functions in a given period of time over predecessor chips. This results in an increase in the electrical current used by these integrated circuit chips. As a result, these integrated circuit chips generate more ohmic heat than the predecessor chips. Accordingly, heat management has become a primary concern in the development of electronic devices.
Typically, heat generating sources or devices, such as integrated circuit chips, are mated with heat sinks to remove heat which is generated during their operation. However, thermal contact resistance between the source or device and the heat sink limits the effective heat removing capability of the heat sink. During assembly, it is common to apply a layer of thermally conductive grease, typically a silicone grease, or a layer of a thermally conductive organic wax to aid in creating a low thermal resistance path between the opposed mating surfaces of the heat source and the heat sink. Other thermally conductive materials are based upon the use of a binder, preferably a resin binder, such as a silicone, a thermoplastic rubber, a urethane, an acrylic, or an epoxy, into which one or more thermally conductive fillers are distributed.
Typically, these fillers are one of two major types: thermally conductive, electrically insulative or thermally conductive, electrically conductive fillers. Aluminum oxide, magnesium oxide, zinc oxide, aluminum nitride, and boron nitride are the most often cited types of thermally conductive, electrically insulative fillers used in thermal products. Boron nitride, and, more specifically, hexagonal boron nitride (hBN) is especially useful in that it has excellent heat transfer characteristics and is relatively inexpensive.
For fillers, it is desirable to achieve as high a thermal conductivity (or as low a thermal resistant) as possible. In order to achieve sufficient thermal conductivity with presently used fillers, such as hBN, it is desirable to employ high loadings of filler in the binder. However, because of the flaky (platelet) structure of hBN particles, achieving solids loading higher than 20 vol. % becomes difficult.
U.S. Pat. Nos. 5,898,009, 6,048,511, and European Patent No. EP 0 939 066 A1, all to Shaffer et al., teach an alternate methodology to further improve solids hBN loading. This involves: (a) cold pressing crushed hBN powder, (b) breaking the cold pressed compact into smaller pieces, and (c) screening the resulting pieces to achieve agglomerates in a desired size range. These agglomerates, however, are non-spherical (angular shape) with jagged short edges. This shape is not ideal for optimizing solids loading due, primarily, to the following reasons: (1) non-spherical shaped agglomerates do not slide against each other easily, thus raising the viscosity; and (2) non-spherical shaped agglomerates have higher surface area and hence absorb greater amounts of polymer on their surface which results in lower amounts of free available polymer, thus, once again raising the viscosity.
Thus, there is a need for thermally conductive filler materials which can be used at high loading levels to achieve sufficient thermal conductivity without increasing viscosity. The present invention is directed to overcoming this deficiency in the art.